Trussed rafters or roof trusses are now specified for the majority of domestic roofs constructed in the UK and Eire. The trussed rafter is an individually designed component, engineered to provide a structural frame for the support of a roof or similar structures.
Pre-fabricated from high quality, stress graded timbers and joined with steel nail-plate fasteners, the trussed rafter offers:
A flexible and practical fully engineered solution to your roofing requirements.
Economy of materials, as trussed rafters can use up to 40% less timber than a traditionally formed roof.
Reduced labour costs on site, due to the amount of prefabrication, releasing site joiners for more complex areas.
Quick erection of the roof structure, enabling other trades to commence quickly.
Reduction in site waste, loss and pilferage of valuable materials.
Space saving on site, with no need for timber storage or carpentry work areas.
Why choose trussed rafters?
Trussed rafters are individually designed components made from kiln-dried, strength-graded timber joined together with steel nailplates. They provide a structural framework to support the roof fabric, ceilings and /or floors.
Trussed rafters can be used on a wide range of building types including timber frame, masonry and steel frame. Although most commonly used for domestic housing they are increasingly chosen for commercial and industrial buildings such as offices, shops, hotels and leisure centres.
Trussed rafters stand out as a sophisticated and highly-engineered product, with superior fabrication underpinned by high production standards. They are more flexible and more cost effective than many other methods of roof construction.
Benefits of Trussed Rafters
Suitable for a wide range of roof structures and building types
Speed of construction allows just-in-time delivery to site
Space saving – no on-site storage area required
Prefabricated components mean reduced labour costs
Up to 40% more energy efficient than traditional construction methods
More thermally efficient than alternative materials such as aluminium, steel or concrete
Made of timber, the only truly renewable building resource
Low-tech manufacturing process reduces emissions and pollution
Minimal environmental impact from installation
Trussed rafters – a versatile solution
Trussed rafters play an increasingly important role in small to medium sizes commercial buildings such as hotels, schools, supermarkets etc. Architectural preference for pitched roofs, the ability to produce long spans, and the fact that they can be used on timber frame, masonry and steel framed buildings increasingly means that trussed rafters provide an ideal solution to roofing commercial buildings.
For more examples of the versatility of trussed rafters and the range of projects they for which they can be used, have a look at the Image Gallery.
Minimum 100 mm mineral wool 48kg/m3 min density packed between joists on wire mesh support stapled at least 50 mm up the side of joists to give fire resistance equivalent to floor and to cover punched metal plate fasteners at the floor level. Additional insulation may be required for thermal resistance e.g. add a layer of lower density mineral wool.
37 mm is the absolute minimum thickness required where joists form part of fire resisting constructions. Joists for ‘RiR’ trussed rafters will normally be nominally 47 mm thick
Room in Roof (RiR) Trussed Rafters
With the ever increasing demand to optimise building land, more and more builders are looking to trussed rafters to provide the solution for additional living areas within the roof space.
A significant proportion of all trussed rafters now produced are RiR trusses which offer up to 65% more living space using a method of construction that has been in common use within the construction industry for some 40 years or so. No outdated or untried methods of construction are needed.
For most purposes ‘RiR’ trussed rafters can be designed to clear span between the front and rear walls of a dwelling thus avoiding the need for building loadbearing walls and foundations on lower storeys. However, if loadbearing walls exist or can easily be added then they can be used to good effect to provide additional support to the ‘RiR’ trusses. In this way greater room sizes are possible but to be effective the walls should occur within the centre 20% of the truss span and are most effective when placed near the mid-span of the truss.
Size of ‘RiR’ Trussed Rafters
Where possible keep the size of ‘RiR’ trussed rafters within the limits dictated by safe transportation. There may be local conditions that affect this but generally an overall height of truss of 4 metres is easy to transport. If greater height is required then trusses may be constructed in two parts. The two-part trusses will be structurally joined on site and instructions for this will normally be provided by the trussed rafter fabricator. This joint is often made with a proprietary connector plate. Fig. 5 shows a typical two-part truss arrangement.
Services in ‘RiR’ Trussed Rafters
The lower void area in ‘RiR’ trusses is an ideal location for services, allowing lateral runs to be positioned between the bottom chords of the trusses. Access to this void area and the service runs can be made via a small hatch in the low level partition.
The lower member of the truss forms not only the floor joist for the attic room but it also makes a vital contribution to the stability of the whole roof. Under no circumstances should the floor joist of a ‘RiR’ trussed rafter be notched or drilled to accommodate services.
Layout of ‘RiR’ Trussed Rafters & Planning Position of Openings
The application of a few basic principles at the concept stage of a project can often result in substantial cost savings by maximising the use of prefabricated components and minimising loose infill areas. Try to locate opening in the roof to fit in with the normal spacing of ‘RiR’ trussed rafters (usually 600 mm). This can often result in reducing the number of trussed rafters required.
Dormer windows and stairwell openings are formed by placing multiple trusses either side of the openings and framing the resulting space with loose timbers. Placing stairwells parallel to truss spans and ensuring that windows are positioned opposite each other will make the overall roof design simpler and cheaper.
Bracing of ‘RiR’ Trussed Rafters
In common with all other trussed rafter roofs, ‘RiR’ trussed rafters need to be braced. Special attention must be given to diagonal bracing of the rafters since the space beneath them will form part of the habitable space of the roof.
It is not possible to provide bracing details within the scope of this Product Data Sheet but full details are given in ‘Technical Handbook – Site Installation Guide’, available from the Trussed Rafter Association priced at £.
Water Tanks in ‘RiR’ Trussed Rafter Roofs
Full details of supporting water tanks in ‘RiR’ trusses are given in the ‘Technical Handbook -Site Installation Guide.
Thermal Insulation
Thought should be given at an early stage to the type and position of the thermal insulation since this could affect the size of rafter required.
Shown in Fig. 9 is an arrangement for insulation within the roof of a ‘Room in Roof” trussed rafter construction. An alternative arrangement is shown in the full downloadable version of the document.
In both cases an air gap of 50 mm should be provided between the top of the insulation and the underside of the roof covering. In addition eaves level vents equivalent to a continuous 25 mm gap must be provided.
Fire Resistance
Under normal circumstances dwelling roofs are not required to have fire resistance under UK Building Regulations. However, ‘RiR’ trussed roofs are slightly different since the ceiling tie of the truss forms the floor of the upper storey and, therefore, are controlled in exactly the same way as any other intermediate house floor.
Where the ‘RiR’ forms the second storey of a dwelling then the floor must provide a ‘modified’ 30 mins fire resistance. In the case where the roof space forms the third storey then the floor must provide a full 30 mins fire resistance.
The only formal guidance on roof construction is given in Approved Document ‘Timber Intermediate Floors for Dwellings’ published by TRADA. Fig. 11 shows the principle recommendations of the TRADA AD.
TRADA recommend a simpler form of construction that satisfies both the ‘modified’ and full 30 minute fire resistance requirements by employing a thicker or higher grade of plasterboard to the ceiling beneath the floor and within the roof space. Details are given in the full downloadable version of the document.
Downloads
PDF version of ‘RiR’ Leaflet(175kb)
Product Data Sheet 1 – RiR Trussed Rafters(682kb)
Product Data Sheet 8 – Loft Conversions with Trussed Rafter Roofs(407kb)
